Mixing chamber

ABSTRACT

A membrane module ( 5 ) including a plurality of porous membranes ( 6 ) extending in an array and mounted, at least at one end, in a header ( 8 ). The header ( 8 ) has a number of distribution apertures ( 11 ) for distributing a fluid into the module ( 5 ) and along a surface or surfaces of the membranes ( 6 ). An elongate chamber ( 10 ) having one open end ( 13 ) and another end is in fluid communication with the distribution apertures ( 11 ) for distributing the fluid to the distribution apertures ( 11 ).

TECHNICAL FIELD

The present invention relates to apparatus and related methods for useof a chamber in association with membrane filtration modules to provideimproved fluid distribution and flow into the associated modules.

BACKGROUND OF THE INVENTION

The importance of membranes for treatment of waste water is growingrapidly. It is now well known that membrane processes can be used as aneffective tertiary treatment of sewage and provide quality effluent.However, the capital and operating cost can be prohibitive. With thearrival of submerged membrane processes where the membrane modules areimmersed in a large feed tank and filtrate is collected through suctionapplied to the filtrate side of the membrane, membrane bioreactorscombining biological and physical processes in one stage promise to bemore compact, efficient and economic. Due to their versatility, the sizeof membrane bioreactors can range from household (such as septic tanksystems) to the community and large-scale sewage treatment.

The success of a membrane filtration process largely depends onemploying an effective and efficient membrane cleaning method. Commonlyused physical cleaning methods include backwash (backpulse, backflush)using a liquid permeate or a gas, membrane surface scrubbing or scouringusing a gas in the form of bubbles in a liquid. Examples of the secondtype of method is illustrated in U.S. Pat. No. 5,192,456 to Ishida etal, U.S. Pat. No. 5,248,424 to Cote et al, U.S. Pat. No. 5,639,373 toHenshaw et al, U.S. Pat. No. 5,783,083 to Henshaw et al and our PCTApplication No. WO98/28066.

In the examples referred to above, a gas is injected, usually by meansof a pressurised blower, into a liquid system where a membrane module issubmerged to form gas bubbles. The bubbles so formed then travel upwardsto scrub the membrane surface to remove the fouling substances formed onthe membrane surface. The shear force produced largely relies on theinitial gas bubble velocity, bubble size and the resultant of forcesapplied to the bubbles. The fluid transfer in this approach is limitedto the effectiveness of the gas lifting mechanism. To enhance thescrubbing effect, more gas has to be supplied. However, this method hasseveral disadvantages: it consumes large amounts of energy, possiblyforms mist or froth flow reducing effective membrane filtration area,and may be destructive to membranes. Moreover, in an environment of highconcentration of solids, the gas distribution system may graduallybecome blocked by dehydrated solids or simply be blocked when the gasflow accidentally ceases.

For most capillary membrane modules, the membranes are flexible in themiddle (longitudinal direction) of the modules but tend to be tighterand less flexible towards to both potted heads. When such modules areused in an environment containing high concentrations of suspendedsolids, solids are easily trapped within the membrane bundle, especiallyin the proximity of two potted heads. The methods to reduce theaccumulation of solids include the improvement of module configurationsand flow distribution when gas scrubbing is used to clean the membranes.

Our earlier International Application No. WO 00/18498 describes the useof a mixture of gas and liquid to effectively clean the surface ofmembranes. The arrangements and methods described herein providedanother simple way of achieving effective scouring of membrane surfaces.

DISCLOSURE OF THE INVENTION

The present invention, at least in its embodiments, seeks to overcome orleast ameliorate some of the disadvantages of the prior art or at leastprovide the public with a useful alternative.

According to one aspect the present invention provides a membrane moduleincluding a plurality of porous membranes extending in an array andmounted, at least at one end, in a header, said header having a numberof distribution apertures for distributing a fluid into said module andalong a surface or surfaces of said membranes, a chamber having one openend and another end in fluid communication with said distributionapertures for distributing said fluid to said distribution apertures.

In an alternative aspect, the present invention provides an assembly ofmembrane modules including a plurality of porous membranes extending inan array and mounted, at least at one end, in a plurality of respectiveheaders, said headers being configured to provide a number ofdistribution apertures therebetween for distributing a fluid into saidassembly of membrane modules and along a surface or surfaces of saidmembranes, a chamber having one open end and another end in fluidcommunication with said distribution apertures for distributing saidfluid to said distribution apertures.

In one form of the invention, the fluid may be gas, usually air and inanother form of the invention the fluid may be a mixture of gas andliquid, usually air and feed liquid.

The term liquid as used herein will be familiar to those skilled in theart as encompassing the range of other materials usually considered asliquid feeds, such as suspensions which contain suspended solids orinorganic matter in liquids, suspensions of biomass in water, waterwhich is turbid and the like, or mixtures of these.

Preferably, the chamber is elongate, that is, preferably, the length ofsaid chamber is greater than that required to provide a static head,when the membrane is immersed in a liquid and gas introduced into thechamber, equivalent to the head loss for the gas to flow to saiddistribution apertures. That is, the length of the chamber should besufficient that all gas flows from the supply source or manifold throughthe distribution apertures rather than the open end of the chamber.

While the term mixing chamber is used, it would also be possible todescribe the present invention as a mixing junction.

In some embodiments, the chamber is enclosed on all sides. However, ifthe chamber is sufficiently dimensioned, it may not be necessary for thesides to be enclosed. By way of example only, if the membrane module oran array of modules is in the form of a linear array, with a pluralityof headers, then it may be sufficient just for the chamber to beenclosed along the two longest sides. Preferably, the membrane module isin the form of an extended linear array wherein the chamber has enclosedlong sides. More preferably, the membrane module is in the form of anextended linear array wherein the chamber has unenclosed short sides.

In yet a further alternative, the chamber may have sides but no top. Insuch a case, the sides of the chamber are positioned to substantiallyform a skirt below the header or group of headers. In such a case, thesides of the chamber may not be parallel, but, for example, may slopeinwardly towards the header.

The chamber can be of any shape as desired to contain any configurationof membrane modules. In preferred embodiments, the header or headers aremounted in a clover shaped manifold. The clover manifold is so calledbecause when viewed from above, the manifold has the shape of a cloverleaf. While the invention is described with reference to this onepreferred embodiment, it will be understood that the manifold can beconfigured to have any desired footprint, for example, it may be linear,rectangular, square, hexagonal etc.

According to another aspect, the present invention provides a method ofremoving a fouling material from a plurality of porous hollow fibermembranes mounted and extending longitudinally in an array to form amembrane module, the method comprising the steps of:

-   providing a source of gas to a chamber in fluid communication with    said membrane module;-   flowing the gas from the chamber into a base of the membrane module    to form gas bubbles therein when said module is immersed in a    liquid, whereby an upward flow of the gas bubbles across surfaces of    the hollow fiber membranes is obtained, and whereby fouling    materials are dislodged from the surfaces of the porous hollow fiber    membranes.

The source of gas can be provided to the chamber either within thechamber itself, or from below the chamber.

Preferably, said chamber is elongate with one end open and the other endin fluid communication with the membrane module. For preference, the gasis provided through the open end of the chamber.

According to another aspect, the present invention provides a method ofremoving a fouling material from a plurality of porous hollow fibermembranes mounted and extending longitudinally in an array to form amembrane module, the method comprising the steps of:

-   forming a mixture of gas bubbles and liquid within a mixing chamber;-   injecting the mixture into a base of the membrane module, whereby an    upward flow of the mixture across surfaces of the hollow fiber    membranes is obtained, and whereby fouling materials are dislodged    from the surfaces of the porous hollow fiber membranes.

For preference, the step of forming a mixture includes entraining thegas bubbles into a liquid stream. Preferably, the gas bubbles areentrained into said liquid stream by means of the chamber. For furtherpreference, the gas bubbles are entrained or injected into said liquidstream by means of devices which forcibly mix gas into a liquid flow toproduce a mixture of liquid and bubbles, such devices including a jet,nozzle, ejector, eductor, injector or the like. The gas used may includeair, oxygen, gaseous chlorine or ozone. Air is the most economical forthe purposes of scrubbing and/or aeration. Gaseous chlorine may be usedfor scrubbing, disinfection and enhancing the cleaning efficiency bychemical reaction at the membrane surface. The use of ozone, besides thesimilar effects mentioned for gaseous chlorine, has additional features,such as oxidising DBP (disinfection by-product) precursors andconverting non-biodegradable NOM's (natural organic matters) tobiodegradable dissolved organic carbon.

It is generally preferred if the air entering the mixing chamber isdeflected away from the source of the liquid which is entering themixing chamber. Preferably, the air entering the mixing chamber isdeflected, for example, by way of a T-piece or baffle. The liquidpreferably enters the mixing chamber by way of a nozzle.

According to a further aspect, the present invention provides a membranemodule comprising a plurality of porous membranes, said membranes beingarranged in close proximity to one another, a mixing chamber in fluidcommunication with said module for mixing together liquid and gasbubbles to provide a cleaning mixture and means for flowing saidcleaning mixture along the surface of said membranes to dislodge foulingmaterials therefrom.

According to one preferred form, the present invention provides a methodof removing fouling materials from the surface of a plurality of poroushollow fibre membranes mounted and extending longitudinally in an arrayto form a membrane module, said membranes being arranged in closeproximity to one another, the method comprising the steps of forming amixture of gas bubbles and liquid within a mixing chamber, said mixturebeing formed by said gas bubbles being entrained in said liquid byflowing said liquid past a source of gas so as to cause said gas to bedrawn and/or mixed into said liquid, flowing said mixture into saidmembrane module such that said bubbles pass substantially uniformlybetween each membrane in said array to, in combination with said liquidflow, scour the surface of said membranes and remove accumulated solidsfrom within the membrane module.

For preference, the membranes comprise porous hollow fibres, the fibresbeing fixed at each end in a header, the lower header having one or moreholes formed therein through which mixture of gas/liquid is introducedfrom the mixing chamber. The holes can be circular, elliptical or in theform of a slot.

Preferably, the membranes comprise porous hollow fibres, the fibresbeing fixed at each end in a plurality of headers, the lower headersbeing configured to provide a number of distribution aperturestherebetween through which mixture of gas/liquid is introduced from themixing chamber.

The fibres are normally sealed at the lower end and open at their upperend to allow removal of filtrate, however, in some arrangements, thefibres may be open at both ends to allow removal of filtrate from one orboth ends. It will be appreciated that the cleaning process described isequally applicable to other forms of membrane such flat or platemembranes.

Alternatively, the membranes may be flat sheet or curtain like hollowfibre modules, with apertures in the header configured parallel to theflat sheet.

In yet a further alternative embodiment, a plurality of headers withoutapertures may be used, provided these are spaced such that the gapsbetween the headers define an aperture or apertures for the fluid andgas bubbles to scrub the membranes.

In an example of this alternative aspect, the membrane module includes aplurality of porous membranes extending in an array and potted inheaders. Said modules are mounted in such a way that said headers areconfigured to provide a number of distribution apertures therebetweenfor distributing a fluid into said modules and along surfaces of saidmembranes, a chamber having one open end and another end in fluidcommunication with said distribution apertures for distributing saidfluid to said distribution apertures.

Particularly in the case of flat-sheet membranes or curtain-like hollowfiber modules, where there are no apertures are in the lower header,apertures or passages for fluid and gas bubbles can be formed bymounting modules in close proximity leaving a gap or gaps betweenmodules.

A mixing chamber can enclose several modules in an array.

According to yet a further aspect, the present invention provides amembrane module for use in a membrane bioreactor including a pluralityof porous hollow membrane fibres extending longitudinally between andmounted at each end to a respective potting head, said membrane fibresbeing arranged in close proximity to one another, said fibres beingpartitioned into a number of bundles at least at or adjacent to theirrespective potting head so as to form a space therebetween, a mixingchamber connected or open to a source of gas and liquid, one of saidpotting heads having an array of openings formed therein in fluidcommunication with said chamber for providing gas bubbles within saidmodule such that, in use, said bubbles move past the surfaces of saidmembrane fibres to dislodge fouling materials therefrom.

According to a further aspect, the invention provides a membrane modulefor use in a membrane bioreactor including a plurality of porous hollowmembrane fibres extending longitudinally between and mounted at each endto a plurality of respective potting heads, said membrane fibres beingarranged in close proximity to one another, said fibres beingpartitioned into a number of bundles at least at or adjacent to theirrespective potting head so as to form a space therebetween, a mixingchamber connected or open to a source of gas and liquid, said pottingheads being configured to provide a number of distribution aperturestherebetween in fluid communication with said chamber for providing gasbubbles within said module such that, in use, said bubbles move past thesurfaces of said membrane fibres to dislodge fouling materialstherefrom.

The liquid used may be the feed to the membrane module. The fibresand/or fibre bundles may cross over one another between the pottingheads though it is desirable that they do not.

Preferably, the fibres within the module have a packing density (asdefined above) of between about 5 to about 70% and, more preferably,between about 8 to about 55%.

For preference, said holes have a diameter in the range of about 1 to 40mm and more preferably in the range of about 1.5 to about 25 mm. In thecase of a slot or row of holes, the width of slots are chosen to beequivalent to the diameter of the above holes.

Typically, the fibre inner diameter ranges from about 0.1 mm to about 5mm and is preferably in the range of about 0.25 mm to about 2 mm. Thefibres wall thickness is dependent on materials used and strengthrequired versus filtration efficiency. Typically wall thickness isbetween 0.05 to 2 mm and more often between 0.1 mm to 1 mm.

For preference, the membrane modules of the present invention include adeflector within said mixing chamber configured to deflect gas away fromthe source of the liquid. It is also preferred if the membrane modulesof the present invention include a nozzle whereby liquid is introducedinto the mixing chamber.

According to another aspect, the present invention provides a membranebioreactor including a tank having means for the introduction of feedthereto, means for forming activated sludge within said tank, a membranemodule according to other aspects of the present invention positionedwithin said tank so as to be immersed in said sludge and said membranemodule provided with means for withdrawing filtrate from at least oneend of said fibre membranes.

According to yet another aspect, the present invention provides a methodof operating a membrane bioreactor of the type described in the aboveaspect comprising introducing feed to said tank, applying a vacuum tosaid fibres to withdraw filtrate therefrom while periodically orcontinuously supplying a cleaning mixture of gas bubbles and liquidformed in a mixing chamber through said openings to within said modulesuch that, in use, said cleaning mixtures flows along the surface ofsaid membrane fibres to dislodge fouling materials therefrom.

If required, a further source of aeration may be provided within thetank to assist microorganism activity and to reduce anoxic zone. Forpreference, the membrane module is suspended vertically within the tankand said further source of aeration may be provided beneath thesuspended module. Preferably, the further source of aeration comprises agroup of air permeable tubes or discs. The membrane module may beoperated with or without backwash depending on the flux. A high mixedliquor of suspended solids (5,000 to 20,000 ppm) in the bioreactor hasbeen shown to significantly reduce residence time and improve filtratequality. The combined use of aeration for both degradation of organicsubstances and membrane cleaning has been shown to enable constantfiltrate flow without significant increases in transmembrane pressurewhile establishing high concentration of MLSS. The use of partitionedfibre bundles enables higher packing densities to be achieved withoutsignificantly compromising the gas scouring process. This provides forhigher filtration efficiencies to be gained.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:—

FIG. 1 shows a pictorial side elevation of a chamber and membranemodules according to an embodiment of the invention;

FIG. 2 shows a pictorial side elevation of a chamber and membranemodules according to a second embodiment of the invention;

FIG. 3 shows a pictorial side elevation of a chamber and membranemodules according to a third embodiment of the invention;

FIG. 4 shows a pictorial side elevation of a chamber and membranemodules according to a fourth embodiment of the invention;

FIG. 5 shows a pictorial side elevation of a chamber and membranemodules according to a fifth embodiment of the invention; and

FIG. 6 shows a schematic side elevation of a chamber and membrane moduleaccording to a sixth embodiment of the invention.

FIG. 7 shows a pictorial side elevation of a chamber and membranemodules according to another embodiment of the invention.

FIG. 8 a shows a preferred embodiment of the deflector for use in mixingchambers of the present invention.

FIG. 8 b shows a further referred embodiment of the deflector for use inmixing chambers of the present invention.

FIG. 9 shows a preferred embodiment of an extended chamber and lineararray of modules.

PREFERRED EMBODIMENTS OF THE INVENTION

Referring to the drawings, the embodiments of the invention will bedescribed in relation to a membrane module of the type disclosed in ourearlier PCT application Nos. WO98/28066 and WO00/18498 which areincorporated herein by cross-reference, however, it will be appreciatedthat the invention is equally applicable to other forms of membranemodule.

As shown in FIG. 1, the membrane module 5 typically comprises fibre,tubular or flat sheet form membranes 6 potted into a pot 7 which issupported by a header 8. The membranes are typically encased in asupport structure (not shown). In the embodiment shown, the headers 8are coupled to a clover type manifold 9 which in turn is connected to anopen ended elongate chamber 10 positioned below the manifold 9. Themembrane module is typically immersed in a feed tank and either one orboth ends of the membranes may be used for the permeate collection. Thebottom of each membrane module 5 has a number of through apertures 11 inthe pot 7 to distribute gas or a mixture of gas and liquid feed past themembrane surfaces.

FIG. 2 shows an embodiment where the chamber 10 is used to produce aliquid/gas bubble mixture by providing a source of gas 12 within thechamber 10 and flowing feed liquid through the chamber 10 to mix with agas flow or gas bubbles produced from the gas source 12. In thisembodiment the gas is fed from above through the clover manifold 9 asthe membrane modules are typically suspended vertically in a feed tank,however, it will be appreciated that the gas may be provided to thechamber by any desired arrangement. The chamber 10 is open at its base13 and liquid is flowed from a pipe 14 upwardly through the chamber 10to mix with gas provided from a source 12 within the chamber 10. Ifnecessary, a non-return valve (not shown) or the like may be attached tothe gas source 12 to prevent the liquid phase entering the gas manifold.

The two fluids are mixed within the chamber 10 before being fed anduniformly distributed into the membrane modules 5 via the distributionapertures 11. The chamber 10 may be directly connected to a gas source12 and/or liquid or as a capture and mixing device.

Referring to FIG. 3, the chamber is shown in its application as a deviceto capture gas and/or liquid flow injected beneath it at its base 13.The fluid flow energy is therefore concentrated in the chamber 10 beforedistribution into the membrane modules 5. In this arrangement thechamber 10 is again open-ended at its base 13 but gas or liquid isprovided from a source, in this case a pipe 14, below the open end andthe chamber is used to capture the upward flow of these fluids forcommunication to the distribution apertures 11.

A similar embodiment is shown in FIG. 4. In this embodiment, a venturidevice 15 or the like is positioned at the base 13 of the chamber 10.The venturi device 15 intakes gas through inlet 16, mixes or entrainsthe gas with liquid flowing through feed inlet 17, forms gas bubbles anddiffuses the liquid/gas mix into the chamber 10. The liquid/gas mixturepasses upwardly from the chamber 10 into the lower header 8 and throughthe distribution apertures 11. Liquid feed is also drawn through theopen end of the chamber 10 by liquid/gas flow from the venturi device15. The entrained gas bubbles scrub membrane surfaces while travellingupwards along with the liquid flow. Either the liquid feed or the gascan be a continuous or intermittent injection depending on the systemrequirements. With a venturi device it is possible to create gas bubblesand aerate the system without a blower. The venturi device 15 can be aventuri tube, jet, nozzle, ejector, eductor, injector or the like.

Although the embodiments of FIGS. 3 and 4 are shown with an open-endedchamber 10, it will be appreciated that a closed chamber may be usedwith gas and liquid being directly injected into the chamber.

The liquid commonly used to entrain the gas is the feed water,wastewater or mixed liquor to be filtered. Pumping such an operatingliquid through a venturi or the like creates a vacuum to suck the gasinto the liquid, or reduces the gas discharge pressure when a blower isused. By providing the gas in a flow of the liquid, the possibility ofblockage of the distribution apertures 11 is substantially reduced.

The arrangement shown in the embodiment of FIG. 5 also serves to reducethe likelihood of blockage of the distribution apertures 11 by largeparticles. In this arrangement gas, typically air, is injected into theclover manifold 9 and the chamber 10 is lengthwise dimensioned to begreater than that required to provide a static head, when the membraneis immersed in a liquid and gas introduced into the chamber 10,equivalent to the head loss for the gas to flow to said distributionapertures 11. As can be seen from the figure, as gas enters from aboveit forces the liquid within the chamber 10 downwards until the gasflowing through the distribution apertures 11 equalizes the pressurewithin the chamber 10 and forms a liquid seal 18 to prevent gas passingoutward through the lower open end 13 of the chamber 10. Such anarrangement has been found to prevent large particles within the feedliquid flowing into and blocking the distribution apertures 11. Theselarge particles usually remain within the chamber 10 and settle undergravity following which they can be removed during the usual drain downof the feed tank.

FIG. 6 shows a similar arrangement to FIG. 3 but with a single membranemodule 5. Chamber 10 again captures gas or liquid/gas flow from source12 and distributes the flow to apertures 11 in pot 7. The flow thenpasses upwardly between the membranes 6. In the embodiment shownfiltrate is withdrawn from the upper header 19 and a screen 20 isprovided between the headers to support the membranes 6.

FIG. 7 shows a further embodiment of the invention in which gas orliquid/gas flow from source 12 is deflected within chamber 10 by meansof a deflector 30. The deflector may be, for instance, a T-piece or moreparticularly a baffle. The deflector preferably functions to prevent theflow 12 from interfering with the flow of air or liquid from source 14.In the particular embodiment shown, the liquid flow into the chamberfrom 14 is via a nozzle 15. The deflector is shown attached to, andpositioned adjacent to, air source 12, however, it could be attached to,and positioned adjacent to nozzle 15. Alternatively, it could be notdirectly attached to either air or gas source, but disposed intermediatethe two.

The use of a nozzle is generally preferred over the use of a sparger.The nozzle is any device which gradually reduces the cross sectionalarea of the throat through which the gas or liquid passes. Nozzles havebeen found particularly advantageous because they can achieve high fluidvelocities with relatively low energy losses. This in turn results inbetter mixing.

FIG. 8 shows one particular form of deflector according to the presentinvention.

FIG. 9 shows a particular embodiment of the invention which is suitablefor scrubbing a linear array of modules. A plurality of arrays areconnected to a mixing chamber 10 of extended length. The gas manifold 12is disposed below the mixing chamber, and the liquid source 14 isdisposed below the gas manifold. A nozzle 15 is preferably used. Theliquid and gas are mixed in or below the chamber and exit via apertures11, scrubbing fibres 6 as they move upwards.

It will be appreciated that further embodiments and exemplifications ofthe invention are possible without departing from the spirit or scope ofthe invention described.

1. A membrane filtration apparatus comprising: a plurality of membranefiltration modules, each membrane filtration module comprising: aplurality of porous membranes extending in an array, said plurality ofporous membranes encased in a support structure and having lower endsmounted in a lower pot supported by a lower header and upper endsmounted in an upper pot supported by an upper header, said upper headerconfigured to provide for permeate to be withdrawn from said upper endsof said plurality of porous membranes; and a plurality of distributionapertures defined in said lower pot, said distribution aperturesconfigured to distribute a scrubbing fluid into said module and along asurface or surfaces of said membranes; a single manifold coupled to saidlower header of each of said plurality of membrane filtration modules;and a single chamber positioned below, and connected to, said manifold,said chamber constructed and arranged to promote upward flow of feedliquid therethrough, said chamber comprising: an open base end in fluidcommunication with a source of feed liquid; a second end in fluidcommunication with said distribution apertures; and a single gas inletconstructed and arranged to introduce gas into said chamber in adownward direction from above the open base end, said gas fed from aboveand through said manifold and into said chamber, said gas inlet centeredbetween at least two of said plurality of membrane filtration modulesand configured to release gas into said chamber at a position verticallydisplaced below said at least two of said plurality of membranefiltration modules, said chamber configured to mix gas and liquid toproduce said scrubbing fluid and further configured to distribute saidscrubbing fluid to said distribution apertures.
 2. The membranefiltration apparatus according to claim 1 wherein the chamber iselongate.
 3. The membrane filtration apparatus according to claim 1wherein the length of said chamber is greater than that required toprovide a static head, when the membrane is immersed in a liquid and gasintroduced into the chamber, equivalent to the head loss for the gas toflow to said distribution apertures.
 4. The membrane filtrationapparatus according to claim 1 wherein the chamber is enclosed on allsides.
 5. The membrane filtration apparatus according to claim 1 whereinthe chamber comprises a plurality of sides positioned to form a skirtdirectly beneath a header or plurality of headers.
 6. The membranefiltration apparatus according to claim 1 wherein said plurality ofmembrane filtration modules are arranged in the form of an extendedlinear array, and wherein the chamber has enclosed long sides.
 7. Themembrane filtration apparatus according to claim 6 wherein the chamberhas unenclosed short sides.
 8. An assembly of membrane modulescomprising: a plurality of porous membranes extending in an array andhaving lower ends mounted in a plurality of lower pots supported by aplurality of respective lower headers, and upper ends mounted in aplurality of upper pots supported by a plurality of respective upperheaders, said lower pots being configured to provide a number ofdistribution apertures therein for distributing a scrubbing fluid intosaid assembly of membrane modules and along a surface or surfaces ofsaid membranes, said lower headers coupled to a manifold; and a chamberpositioned below and connected to said manifold, said chamberconstructed and arranged to promote upward flow of feed liquidtherethrough, said chamber comprising: an open base end in fluidcommunication with a source of feed liquid; a second end in fluidcommunication with said distribution apertures; and a gas inletconstructed and arranged to introduce gas into said chamber in adownward direction from above the open base end, said gas fed from aboveand through said manifold, said chamber configured to mix gas and liquidto produce said scrubbing fluid and further configured to distributesaid scrubbing fluid to said distribution apertures.
 9. The assembly ofmembrane modules according to claim 8 wherein the chamber is elongate.10. The assembly of membrane modules according to claim 8 wherein thelength of said chamber is greater than that required to provide a statichead, when the membrane is immersed in a liquid and gas introduced intothe chamber, equivalent to the head loss for the gas to flow to saiddistribution apertures.
 11. The assembly of membrane modules accordingto claim 8 wherein the chamber is enclosed on all sides.
 12. Theassembly of membrane modules according to claim 8 wherein the chambercomprises a plurality of sides positioned to form a skirt directlybeneath a header or plurality of headers.
 13. The assembly of membranemodules according to claim 8 when arranged in the form of an extendedlinear array wherein the chamber has enclosed long sides.
 14. Theassembly of membrane modules according to claim 8 in the form of anextended linear array wherein the chamber has unenclosed short sides.15. A membrane filtration apparatus comprising: a plurality of membranefiltration modules, each membrane filtration module comprising aplurality of porous membranes, said membranes being arranged in closeproximity to one another and having lower ends mounted in a lower potsupported by a lower header and upper ends mounted in an upper potsupported by an upper header, said upper header configured to providefor permeate to be withdrawn from said upper ends of said porousmembranes; a manifold coupled to said lower headers; an open-endedmixing chamber constructed and arranged to provide a cleaning mixture bymixing together liquid and gas bubbles, said chamber immersed in a feedtank and having an open base in fluid communication with a source offeed liquid, said chamber constructed and arranged to promote upwardflow of feed liquid therethrough; a gas source positioned within theopen-ended mixing chamber, the gas source constructed and arranged tointroduce gas through a single gas inlet into the open-ended mixingchamber in a downward direction from above the open base, said gas fedfrom above and through said manifold and into said chamber, said singlegas inlet centered within said plurality of membrane modules; and meansfor flowing said cleaning mixture along a surface of said membranes todislodge fouling materials therefrom.
 16. A membrane bioreactorcomprising: a plurality of membrane filtration modules, each membranefiltration module comprising a plurality of porous hollow membranefibres extending longitudinally between and mounted between an upper anda lower potting head, said membrane fibres being arranged in closeproximity to one another, said fibres being partitioned into a number ofbundles at least at or adjacent to their respective potting head so asto form a space therebetween; a header in which the lower potting headis supported; a manifold coupled to the header; an open-ended mixingchamber positioned below the lower potting head, said chamberconstructed and arranged to promote upward flow of feed liquidtherethrough, said chamber having an open base in fluid communicationwith a source of feed liquid; and a gas inlet positioned within theopen-ended mixing chamber, the gas inlet spaced from and surrounded byside walls of the open-ended mixing chamber and configured to feed gasinto the open-ended mixing chamber from above and through said manifold,wherein at least one of said potting heads includes an array of openingsformed therein in fluid communication with said chamber constructed andarranged to provide gas bubbles within said module such that, in use,said bubbles move past the surfaces of said membrane fibres to dislodgefouling materials therefrom.
 17. An assembly of membrane modules for usein a membrane bioreactor comprising: a plurality of porous hollowmembrane fibres extending longitudinally between and mounted between anupper and a lower potting head, said membrane fibres being arranged inclose proximity to one another, said fibres being partitioned into anumber of bundles at least at or adjacent to their respective pottinghead so as to form a space therebetween; a header in which the lowerpotting head is supported; a manifold coupled to the header; anopen-ended mixing chamber positioned below the lower potting head, saidchamber constructed and arranged to promote upward flow of feed liquidtherethrough, said chamber having an open base in fluid communicationwith a source of feed liquid; and a gas inlet positioned within theopen-ended mixing chamber, the gas inlet spaced from and surrounded byside walls of the open-ended mixing chamber, and centrally locatedwithin the open-ended mixing chamber and configured to feed gas into theopen-ended mixing chamber from above and through said manifold; whereinsaid potting heads are configured to provide a number of distributionapertures therebetween in fluid communication with said chamber forproviding gas bubbles within said assembly of membrane modules suchthat, in use, said bubbles move past the surfaces of said membranefibres to dislodge fouling materials therefrom.
 18. The assembly ofmembrane modules according to claim 17 wherein the liquid used is feedto the membrane module.
 19. The assembly of membrane modules accordingto claim 17 wherein the fibres within the module have a packing densityof between about 5 to about 70%.
 20. The assembly of membrane modulesaccording to claim 19 wherein the packing density is between about 8 toabout 55%.
 21. The assembly of membrane modules according to claim 17wherein said holes have a diameter in the range of about 1 to 40 mm. 22.The assembly of membrane modules according to claim 21 wherein saidholes have a diameter in the range of about 1.5 to about 25 mm.
 23. Theassembly of membrane modules according to claim 17 comprising adeflector within said mixing chamber configured to deflect gas away fromthe source of the liquid.
 24. The assembly of membrane modules accordingto claim 17 including a nozzle whereby liquid is introduced into themixing chamber.
 25. A membrane bioreactor comprising a tank having meansfor the introduction of feed thereto, means for forming activated sludgewithin said tank, a membrane module or an assembly according to claim 17positioned within said tank so as to be immersed in said sludge and saidmembrane module provided with means for withdrawing filtrate from atleast one end of said fibre membranes.
 26. A method of operating amembrane bioreactor of the type according to claim 25, comprisingintroducing feed to said tank, applying a vacuum to said fibres towithdraw filtrate therefrom while periodically or continuously supplyinga cleaning mixture of gas bubbles and liquid formed in a mixing chamberthrough said openings to within said module such that, in use, saidcleaning mixtures flows along the surface of said membrane fibres todislodge fouling materials therefrom.
 27. A membrane bioreactoraccording to claim 25 wherein a further source of aeration is providedwithin the tank to assist microorganism activity.
 28. A membranebioreactor according to claim 27 wherein the membrane module issuspended vertically within the tank and said further source of aerationis provided beneath the suspended module.
 29. A membrane bioreactoraccording to claim 28 wherein the further source of aeration comprises agroup of air permeable tube.
 30. The membrane filtration apparatus ofclaim 1 wherein said gas inlet is fluidly connected to a source of gaswithin said chamber.
 31. The membrane filtration apparatus of claim 30wherein said source of gas is coupled to a gas line which runs throughsaid header.
 32. The assembly of membrane modules of claim 8 whereinsaid gas inlet runs through said header.